Australian Federal Police officers during the raid at Raby. A 20-year-old man is one of five people charged with conspiracy to prepare a terrorist act.
Photograph: Nsw Police/PR IMAGE

Police will charge two people arrested on Thursday and another three already in custody with conspiring to prepare a terrorist act, probably an attack on a police building in Sydney.

Ibrahim Ghazzawy, 20, from Raby, and a 15-year-old from Georges Hall who cannot be named were arrested after at least four early-morning raids across Sydney’s south-west.

Another three men already in custody in Goulburn’s Supermax prison — understood to include 20-year-old Sulayman Khalid — will be charged with the same offence.

The charges relate to documents seized in raids on 18 December last year as part of the counter-terrorism investigation Operation Appleby.

“Those documents clearly talked about a plan and a number of government buildings were named in those plans, specifically the AFP building, and as a result of putting all those documents together ... we were able to build a case of conspiracy for five of the people who were involved,” federal police deputy commissioner national security Michael Phelan said.

People more likely to report suspected violent extremists 'if they trust police'

Read more

It is the second allegedly foiled plot to emerge from Appleby, after 22-year-old Omarjan Azari was charged last September with making preparations for a terrorist act, alleged to be the killing of “any random unbeliever”.

Three search warrants were also executed on Thursday morning in Bankstown, Merrylands and Wiley Park. “There were no arrests at those locations,” NSW police deputy commissioner special operations Catherine Burn said.

She said the alleged plot, which was detailed on “pieces of paper” seized in the December 2014 raids, “may have involved an attack on a government building, whether that be AFP or police, we don’t know”.

Khalid was arrested last December allegedly in possession of handwritten notes that police allege describe potential terrorism targets, including government buildings. A court heard in September that fingerprints belonging to a number of different people had been found on the papers.

The teenager, who was 14 years old at the time of the raids, was “part of the conspiracy”, Burn said.

Police had been liaising with him and his school in the past year, but he was not engaged in the federal police’s diversion program for youth at risk of radicalisation.

Burn said three people charged in relation to the murder of police employee Curtis Cheng in September were “associates” of the five people charged on Thursday.

Police will not allege the second plot was directed by overseas members of Islamic State. A court has heard the Azari plot was allegedly concocted at the direction of Mohammad Ali Baryalei, a senior Australian member of Islamic State thought to have been killed last year.

The three men in Supermax were due to be charged later on Thursday.

Ghazzawy was scheduled to appear before Campbelltown local court on Thursday afternoon. The 15-year-old will also appear before Parramatta children’s court on Thursday.

About 800 police were involved in the country’s largest counter-terrorism raids as part of Appleby in September last year.